1. Field of the Invention
The present invention is related to imaging analysis. More particularly, the present invention is related to the use of imaging to perform non-invasive spectral imaging analysis of a subject""s vascular system.
2. Related Art
Most widely accepted methods of blood testing and analysis require invasive and in vitro techniques. For example, a conventional complete blood count including the white blood cell differential (CBC+Diff) test is done in an xe2x80x9cinvasivexe2x80x9d manner in which a sample of venous blood is drawn from a patient through a needle, and submitted to a laboratory for analysis. In addition, it is often necessary to measure other types of blood components, such as non-cellular constituents (e.g., blood gases and bilirubin) present in the plasma component of blood. The most common method for bilirubin analysis is through an in vitro process. In such an in vitro process, a blood sample is invasively drawn from the patient. The formed elements (red blood cells and other cells) are separated by centrifugation and the remaining fluid is reacted chemically and analyzed spectrophotometrically.
Invasive techniques, such as for conventional CBC+Diff tests and bilirubin analysis, pose particular problems for newborns, elderly patients, burn patients, and patients in special care units. Thus it is desirable to utilize a device which is able to rapidly and non-invasively quantitatively measure a variety of blood and vascular characteristics. Such a technique would eliminate the need to draw a venous blood sample to ascertain blood characteristics. A device of this type would also eliminate the delay in waiting for the laboratory results in the evaluation of the patient. Such a device also has the advantage of added patient comfort.
Soft tissue, such as mucosal membranes or unpigmented skin, do not strongly absorb light in the visible and near-infrared, i.e., they do not absorb light in the spectral region where hemoglobin absorbs light. This allows the vascularization to be differentiated from surrounding soft tissue by absorption in a selected spectral range. However, the surface of soft tissue strongly reflects light and the soft tissue itself effectively scatters light after penetration of only 100 microns. Therefore, in vivo visualization of the circulation is difficult because of poor resolution, and generally impractical because of the complexities involved in compensating for multiple scattering and for specular reflection from the surface. The resolution of such images is limited by scattered light. The computations to compensate for this are complex and difficult to implement.
Spectrophotometry involves analysis based on the absorption or attenuation of electromagnetic radiation by matter at one or more wavelengths. The instruments used in this analysis are referred to as spectrophotometers. A simple spectrophotometer includes: a source of radiation, such as, e.g., a light bulb; a means of spectral selection such as a monochromator containing a prism or grating, or a colored filter; and one or more detectors, such as, e.g., photocells, which measure the amount of light transmitted and/or reflected by the sample in the selected spectral region.
In opaque samples, such as solids or highly absorbing solutions, the radiation reflected from the surface of the sample may be measured and compared with the radiation reflected from a non-absorbing or white sample. If this reflectance intensity is plotted as a function of wavelength, it gives a reflectance spectrum. Reflectance spectra are commonly used in matching colors of dyed fabrics or painted surfaces. However, because of its limited dynamic range and inaccuracy, reflection or reflectance spectrophotometry has been used primarily in qualitative rather than quantitative analysis. On the other hand, transmission spectrophotometry is conventionally used for quantitative analysis because Beer""s law (relating the negative logarithm of measured intensity linearly to concentration) can be applied.
Reflectance spectrophotometry is not a primary choice for quantitative analysis because specularly reflected light from a surface limits the available contrast (black to white or signal to noise ratio), and, consequently, the measurement range and linearity. Because of surface effects, measurements are usually made at an angle to the surface. However, only for the case of a Lambertian surface will the reflected intensity be independent of the angle of viewing. Light reflected from a Lambertian surface appears equally bright in all directions (cosine law). However, good Lambertian surfaces are difficult to obtain. Conventional reflectance spectrophotometry presents an even more complicated relationship between reflected light intensity and concentration than exists for transmission spectrophotometry which follows Beer""s law. Under the Kubelka-Munk theory applicable in reflectance spectrophotometry, the intensity of reflected light can be related indirectly to concentration through the ratio of absorption to scattering.
Several devices for in vivo analysis based on reflectance spectrophotometry have been developed recently. However, these conventional reflectance-based devices are less than optimal for several reasons.
For example, one such device uses image analysis and reflectance spectrophotometry to measure individual cell parameters such as cell size. Measurements are taken only within small vessels, such as capillaries where individual cells can be visualized. Because this device takes measurements only in capillaries, measurements made by the device will not accurately reflect measurements for larger vessels. Other devices utilize light application means that focus an illumination source directly onto a blood vessel in a detection region. As a result, these devices are extremely sensitive to movements of the device with respect to the patient. This increased sensitivity to device or patient movement can lead to inconsistent results. To counteract this motion sensitivity, these devices require stabilizing and fixing means.
Other conventional devices have been developed based on a traditional dark field illumination technique. As understood in traditional microscopy, dark field illumination is a method of illumination which illuminates a specimen but does not admit light directly to the objective. For example, a traditional dark field imaging approach is to illuminate an image plane such that the angular distribution of illumination and the angular distribution of light collected by an objective for imaging are mutually exclusive. The illumination is incident on the field of view of the detector, however, so in these devices scattering off optically active tissue in the image path can create an orientation dependent backscatter or image glare that reduces image contrast. Moreover, rotation of these devices causes a change in contrast.
Thus, there is a need for a device that provides for complete non-invasive, in vivo analysis of the vascular system with high image quality. There is a need for a device that provides high resolution visualization of: blood cell components (red blood cells, white blood cells, and platelets); blood rheology; the vessels in which blood travels; and vascularization throughout the vascular system. There is a further need for a device that can minimize the glare and other deleterious artifacts arising in conventional reflectance spectrophotometric systems.
The present invention is directed to a method and apparatus for analysis of a sub-surface object, such as blood or tissue under the skin of a patient, by use of a high contrast illumination technique. In one embodiment, the device includes a light source, an illumination system, and an imaging system. The light source provides an illumination beam that propagates along an illumination path between the light source and the plane in which the object is located (the object plane). The illumination system transforms the illumination beam into a high contrast illumination pattern and projects that illumination pattern onto the sub-surface object. The illumination pattern has a high intensity portion and a low intensity portion. The imaging system includes an image capturing device that detects an image of the sub-surface object.
According to the present invention, the image of the object is formed by scattered illumination from the high contrast illumination pattern that is transmitted through the sub-surface object and propagates along an image path to the image capturing device. Further, the high intensity portion of the illumination pattern is incident on the object plane outside a field of view of the image capturing device.
In a preferred embodiment, the device further includes an illumination pattern generator that transforms the illumination beam into a high contrast illumination pattern. In this embodiment, a relay lens projects the illumination pattern onto the object plane. An obscuration is used to block a portion of the illumination beam. Alternatively, a conical lens (also referred to as an axicon), a conical grating, or a holographic optical element is used to generate a high contrast illumination pattern.
In a further aspect of the present invention, the apparatus includes crossed polarizers that act to prevent any polarized light reflected off the surface of the sub-surface object or reflected off birefringent tissue layers in the near field from reaching the image capturing device.
A further aspect of the present invention provides a method for creating a source of illumination within a sub-surface tissue region that contains an object of interest in a non-invasive manner. The object is illuminated about an object plane wherein the object is located and is detected by an image capturing device. In a first step, a source of light is provided. Next, the light from the source is transformed into a high contrast illumination pattern having a high intensity portion and a low intensity portion. The illumination pattern is directed onto a surface of the tissue region such that the high intensity portion of the illumination pattern is incident upon the object plane outside a field of view of the image capturing device. According to the present invention, the high intensity portion of the illumination pattern undergoes one or more scattering events within the tissue region. Next, the scattered light that interacts with the object is detected by the image capturing device. Scattered light is transmitted through the object, providing aback-illuminated image of the object. The illumination/imaging system works to provide back illumination so that measurements (for example, optical density) can be made as if the measurement was transmission, not reflection.